A conventional trunnion ball valve generally employs a single upstream valve seat to obtain the sealing effect, and a downstream valve seat barely has the sealing effect. A double piston effect for a ball valve mainly uses both the upstream and downstream valve seats to obtain the sealing effect. Even though the upstream valve seat fails to seal the valve, the downstream valve seat can still seal the valve; therefore the sealing effect is effectively improved by this method. This method not only improves the sealing effect of the ball valve but also prolongs the working life of the ball valve. The ball valve with this structure has been used in the petroleum industry for years. However, the application of this technology causes the design of the valve seat to be excessively complicated, the assembling and replacement on site to be difficult and the manufacturing cost to be excessively high.